Background: ω-3 Fatty acids exert several benefits during chemotherapy, such as preventing intestinal mucosal damage and improving response to chemotherapy. However, little is known about the effect of ω-3 fatty acids on chemotherapy-induced hematological toxicities. Methods: Mice that had consumed either an ω-3–rich or an ω–3-poor diet for 2 weeks were intraperitoneally administered cisplatin. The resultant changes in blood cell count, bone marrow cell count, and cytokine levels in bone marrow supernatant were analyzed. The effect of ω-3 fatty acids on human peripheral blood mononuclear cells (PBMCs) exposed to cisplatin was also examined. Results: Although peripheral blood cell counts decreased after cisplatin treatment in both groups of mice, the decrease in white blood cell count was significantly lower in mice that consumed the ω-3–rich diet. The decrease in bone marrow cells after cisplatin treatment was also reduced in mice that consumed the ω-3–rich diet. Levels of stem cell factor (SCF) and fibroblast growth factor 1 (FGF-1) were significantly higher in bone marrow supernatants from mice that consumed the ω-3–rich diet. The rate of apoptosis in PBMCs (after exposure to cisplatin) cultured in medium containing ω-3 fatty acids was significantly lower than in PBMCs cultured in control medium. Conclusion: ω-3–Rich diets reduced chemotherapy-induced leukopenia in mice. This may be the result of increased numbers of bone marrow cells due to higher levels of SCF and FGF-1 in the bone marrow.